Replace upTimeMillis with elapsedRealTime

upTimeMillis does not include time spent in deep sleep. Use
elapsedRealtime instead.

The time spent in deep sleep is important to include since the
keepalives may still be ongoing during deep sleep and so that time
should be included in duration calculations.

Bug: 273451360
Fixes: 281639289
Test: atest FrameworksNetTests
(cherry picked from https://android-review.googlesource.com/q/commit:d5ca3f1f6968a688b2128360da79595d1a1b9784)
Merged-In: I54350d49bdfec58cc00dea6d82cba01f5d387bb0
Change-Id: I54350d49bdfec58cc00dea6d82cba01f5d387bb0